1. Field of the Invention
The present invention relates to a displacement measuring method and apparatus using interference of two beams to measure a displacement or asperity on a measurement surface.
2. Description of the Related Art
Double-beam interferometers are known. These apparatus are used for measuring a step-height difference on a measurement surface. The double-beam interferometer divides coherent light into two beams having the same intensity, and allows one beam to be incident on a reference surface and another beam to be incident on a measurement surface, to produce two reflected beams. The apparatus then measures the optical-path difference (optical-distance difference) between the two reflected beams on the basis of interference fringes produced by the beams. There are various ways of accomplishing these functions, including using an optical dividing technique and an optical-path arrangement technique. Apparatus for implementing these techniques include the Watson type apparatus, the Linnik type apparatus, and the Mirau type apparatus.
According to the conventional double-beam interferometer, when white light is supplied, a range in which a distance to a reference surface dr is equal to a distance to a surface ds to be measured is formed as a zero-order interference fringe, and its position is visually recognized as such. Subsequently, filters are switched, and a monochromatic light is supplied to the double-beam interferometer, and an image of interference fringes for viewing a step-height difference on the surface to be measured is produced. The step-height difference is measured by counting the number of monochromatic-light interference fringes positioned from the zero-order interference fringe to a point to be measured.
As described above, interference fringes caused by white light and interference fringes caused by monochromatic light are not simultaneously obtained. Thus, disadvantageously, a dynamic change (like a displacement on a surface to be measured) cannot be measured, and only static measurements, such as step-height difference measurements, can be performed.